Association, Composition and Aggregation in Java

Association

Association is relation between two separate classes which establishes through their Objects. Association can be one-to-one, one-to-many, many-to-one, many-to-many.
In Object-Oriented programming, an Object communicates to other Object to use functionality and services provided by that object. Composition and Aggregation are the two forms of association.

In this example, there is an Institute which has no. of departments like CSE, EE. Every department has no. of students. So, we make a Institute class which has a reference to Object or no. of Objects (i.e. List of Objects) of the Department class. That means Institute class is associated with Department class through its Object(s). And Department class has also a reference to Object or Objects (i.e. List of Objects) of Student class means it is associated with Student class through its Object(s).
It represents a Has-A relationship.

When do we use Aggregation ??
Code reuse is best achieved by aggregation.

Composition

Composition is a restricted form of Aggregation in which two entities are highly dependent on each other.

It represents part-of relationship.

In composition, both the entities are dependent on each other.

When there is a composition between two entities, the composed object cannot exist without the other entity.

In above example a library can have no. of books on same or different subjects. So, If Library gets destroyed then All books within that particular library will be destroyed. i.e. book can not exist without library. That’s why it is composition.

Aggregation vs Composition

Dependency: Aggregation implies a relationship where the child can exist independently of the parent. For example, Bank and Employee, delete the Bank and the Employee still exist. whereas Composition implies a relationship where the child cannot exist independent of the parent. Example: Human and heart, heart don’t exist separate to a Human

Type of Relationship: Aggregation relation is “has-a” and composition is “part-of” relation.

Type of association: Composition is a strong Association whereas Aggregation is a weak Association.

// Java program to illustrate the
// difference between Aggregation
// Composition.
import java.io.*;
// Engine class which will
// be used by car. so 'Car'
// class will have a field
// of Engine type.
class Engine
{
// starting an engine.
public void work()
{
System.out.println("Engine of car has been started ");
}
}
// Engine class
final class Car
{
// For a car to move,
// it need to have a engine.
private final Engine engine; // Composition
//private Engine engine; // Aggregation
Car(Engine engine)
{
this.engine = engine;
}
// car start moving by starting engine
public void move()
{
//if(engine != null)
{
engine.work();
System.out.println("Car is moving ");
}
}
}
class GFG
{
public static void main (String[] args)
{
// making an engine by creating
// an instance of Engine class.
Engine engine = new Engine();
// Making a car with engine.
// so we are passing a engine
// instance as an argument while
// creating instace of Car.
Car car = new Car(engine);
car.move();
}
}

Output:

Engine of car has been started
Car is moving

In case of aggregation, the Car also performs its functions through an Engine. but the Engine is not always an internal part of the Car. An engine can be swapped out or even can be removed from the car. That’ why we make The Engine type field non-final.

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